1. Field of the Invention
The present invention relates generally to multiple random access satellite communications system in which satellite channels are established between a central station and a plurality of remote stations, and more specifically to a technique for reducing the probability of collisions between simultaneously transmitted packets.
2. Description of the Related Art
The known slotted ALOHA system is a time division multiple access satellite communication system for establishing communications between a central station and a plurality of remote stations. In the slotted ALOHA system where the communication channel is divided into time slots, the remote stations send their packets on a randomly accessed time slot. The beneficial feature of this type of access is the ability to reduce transmission delay when the rate of transmissions is small. However, when the rate of transmissions is high, the rate of collisions increases, resulting in low channel utilization efficiency and long transmission delay. To avoid poor system utilization efficiency as a result of high transmission rate, it is necessary to decrease retransmissions and to avoid collisions between retransmitted packets. A method that is generally employed for retransmission is to use a random number to specify a period of time the conflicting station must wait before an attempt is made to gain access to the channel. If the number of random numbers that can be generated is small, there is a high likelihood of coincidence between two or more remote stations using the same random number. Further, if the rate of retransmissions and hence the rate of collisions is high, it is more effective for the remote stations using a greater range of random numbers to reduce traffic congestion than for those remote stations using a smaller range of random numbers. However, at low retransmission rates conflicting stations may wait an unnecessarily long period of time before they are allowed to retransmit a packet.